The browserWindow size is not the same as the webContents size


This is the screen capture.
I run a example witch built by the electron-vue cli.
I run vue init simulatedgreg/electron-vue my-project to generate the project.
Then cd my-project.
Then npm run dev.
And then this situation happened.
This will happen only in the development mode but not when I build an app package.
Is this a bug?